Hot-rolled steel sheet having excellent surface hardness after carburizing heat treatment and excellent cold workability

ABSTRACT

A hot-rolled steel sheet that: has a sheet thickness of 2-10 mm, contains specific amounts of C, Mn, Al, and N, the remainder being iron and unavoidable impurities, and contains ferrite and pearlite in a specified area ratio, the remainder being bainite. In the hot-rolled steel sheet, a number of grains having an aspect ratio of major axis to minor axis of 3 or less is 60% or more of a number of all grains and an average grain size of the all grains is from 3 to 50 μm.

TECHNICAL FIELD

The present invention relates to a hot-rolled steel sheet exhibiting good cold formability during processing before a heat treatment and exhibiting a predetermined surface hardness and a desired hardness even in a deep portion from the surface after a carburizing heat treatment. More specifically, it relates to, among steel materials used as various structural parts, a hot-rolled steel sheet useful as a material for the manufacture of parts, for example, clutches, dampers, gears, and the like employed in each portion of automobiles, etc, which are subjected to a surface hardening treatment by a carburizing-quenching or carbonitriding-quenching treatment so as to improve the wear resistance and anti-fatigue properties. In the following description, explanations are given by referring typically to a case of application in clutches but the present invention is of course not limited to the manufacture thereof and can be effectively utilized as a material for the manufacture of parts requiring high surface hardness and excellent impact properties by taking advantage of excellent carburizing-quenching performance and carbonitriding-quenching performance thereof to harden the surface portion while maintaining high toughness in the core portion.

BACKGROUND ART

In recent years, from the standpoint of environmental protection, a requirement for weight reduction, i.e., higher strength, of steel materials for use in various parts for automotive, for example, transmission parts such as gears, and casings is more and more increasing with the purpose of enhancing the fuel efficiency of automobiles. To meet this requirement for weight reduction and higher strength, a steel material prepared by hot-forging a steel bar (hot-forged material) has been used as a commonly-employed steel material (for example, see Patent Document 1). In addition, in order to reduce CO₂ emission amount in the process of producing parts, a requirement for cold forging of parts such as gears, which had been heretofore worked by hot forging, is also increasing.

Cold working (cold forging) is advantageous in that the productivity thereof is high compared with hot working and warm working and moreover, both the dimensional accuracy and the steel material yield are good. On the other hand, a problem occurring in the case of producing parts by the cold working is that a steel material having high strength, i.e., high deformation resistance, must be necessarily used so as to ensure that the strength of cold-worked parts is equal to or more than a predetermined value expected. However, a steel material with a higher deformation resistance to be used has a disadvantage of leading to a shortening of the life of a metal mold for cold working.

Under the above-mentioned background, in the field of transmission parts, studies has been carried out to switch from the conventional forged products (e.g., hot-forged and cold-forged) of steel bars to the manufacture of parts using steel sheets, with an aim of achieving weight reduction or cost reduction of parts. Among others, in the parts of which surface is exposed to a contact pressure, such as gears, dampers and clutches, the surface hardness is increased by applying a carburizing heat treatment after machining of a steel sheet into parts, so as to impart wear resistance and anti-fatigue properties. As the steel sheet for the manufacture of these parts, a general soft steel (e.g., SPHC) has been conventionally used, but further higher strength and higher hardness are demanded.

High-strength parts assured of predetermined strength and surface hardness are manufactured by performing a carburizing heat treatment after cold forming (e.g., press forming) of a steel sheet into a predetermined shape. In order to increase the hardness of the carburized surface, it may be thought to increase the amount of a principal component, mainly the C amount, or of an additive element, but this causes a reduction in the cold formability before the heat treatment. Accordingly, a solution capable of achieving both of ensuring the cold formability and enhancing the surface hardness after a carburizing heat treatment has been required.

As described above, the present invention targets a hot-rolled steel sheet. Conventional techniques related to a hot-rolled steel sheet include, for example, the following Patent Documents 2 to 6.

The hot-rolled steel sheet disclosed in Patent Document 2 is supposed to have enhanced balance between strength and stretch flange formability by virtue of a configuration where 70% or more of the metal microstructure in terms of area percentage is a ferrite phase, the average grain size thereof is 50 μm or less, the aspect ratio thereof is 3 or less, 70% or more of the ferrite grain boundary is composed of a large-angle grain boundary, the maximum diameter of the ferrite phase formed of the large-angle grain boundary is 30 μm or less, the area percentage of a precipitate having a minimum diameter of 5 nm or more is 2% or less of the metal microstructure, the average grain size of a second phase having a maximum area percentage among residual phases excluding the ferrite phase and the precipitate is 50 μm or less, and the large-angle grain boundary of the ferrite phase is present between nearest second phases.

The hot-rolled steel sheet disclosed in Patent Document 3 is supposed to have enhanced stretch flange formability by virtue of a configuration where the ferrite average grain size is from 1 to 10 μm, the standard deviation of the ferrite grain size is 3.0 μm or less and the shape ratio of an inclusion is 2.0 or less.

The hot-rolled steel sheet disclosed in Patent Document 4 is supposed to have enhanced stretch flange formability by virtue of a configuration where the microstructure is a ferrite-bainite microstructure having a ferrite phase fraction of 50% or more with the remainder being bainite, and the Mn microsegregation in the range of ⅛t to ⅜t of the sheet thickness t falls within the range satisfying 0.10≧σ/Mn.

The hot-rolled steel sheet disclosed in Patent Document 5 is supposed to have enhanced elongation and stretch flange formability by virtue of a configuration where the microstructure contains a ferrite phase with an area ratio of 20% or more, a tempered martensite phase with an area ratio of 10 to 60%, a martensite phase with an area ratio of 0 to 10%, and a retained austenite phase with a volume percentage of 3 to 15%.

Although the hot-rolled steel sheets disclosed in Patent Documents 2 to 5 are excellent in the cold formability, the surface hardness after a carburizing heat treatment is not referred to at all, and the improvement effect thereon is unknown.

On the other hand, the hot-rolled steel sheet (carburized steel strip) disclosed in Patent Document 6 is supposed to enable reducing the “shear droop” during stamping and omitting a carburizing treatment after the stamping by virtue of a configuration where the average hardness to a depth of 50 μm in the surface layer part in the sheet thickness direction is 170 HV or more, the metal microstructure is ferrite+pearlite, and the difference ΔC=CS−CM between the surface carbon concentration CS (mass %) and the in-steel average carbon concentration CM (mass %) is 0.1 mass % or more.

Although the hot-rolled steel sheet (carburized steel strip) disclosed in Patent Document 6 is excellent in the surface hardness after a carburizing heat treatment, cold formability is not referred to at all, and the improvement effect thereon is unknown.

As described above, almost no studies have been heretofore made on a hot-rolled steel sheet having both cold formability and surface hardness after a carburizing heat treatment.

PRIOR ART LITERATURE Patent Documents

Patent Document 1: Japanese Patent No. 3,094,856

Patent Document 2: Japanese Patent No. 3,821,036

Patent Document 3: Japanese Patent No. 4,276,504

Patent Document 4: Japanese Patent No. 4,644,075

Patent Document 5: JP-A-2011-168861

Paten Document 6: JP-A-2010-222663

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

The object of the present invention is to provide a hot-rolled steel sheet having both cold formability and surface hardness after a carburizing heat treatment. In the present invention, the carburizing heat treatment encompasses also the case of a heat treatment for carbonitridation, in addition to for normal carburization.

Means for Solving the Problems

The invention according to claim 1 is a hot-rolled steel sheet excellent in cold formability and surface hardness after a carburizing heat treatment, having:

a sheet thickness of from 2 to 10 mm;

a component composition containing, in mass % (hereinafter, the same applies to chemical components),

C: from 0.05 to 0.30%,

Mn: from 0.3 to 3.0%,

Al: from 0.015 to 0.1%, and

N: from 0.003 to 0.30%,

with the remainder being iron and unavoidable impurities; and

a microstructure containing, in area ratio,

ferrite: from 10 to 50%,

pearlite: from 15 to 50% and

remainder: bainite,

in which

with respect to grains of all phases including the ferrite and the pearlite (hereinafter, referred to as “all grains”),

a number of grains having an aspect ratio (major axis/minor axis) of 3 or less is 60% or more of a number of the all grains, and an average grain size of the all grains is from 3 to 50 μm.

The invention according to claim 2 is the hot-rolled steel sheet according to claim 1, in which, in the unavoidable impurities, Si is 0.5% or less, P is 0.030% or less and S is 0.035% or less.

The invention according to claim 3 is the hot-rolled steel sheet according to claim 1 or 2, in which the component composition further contains at least any one of the following (a) to (f):

(a) at least one member selected from the group consisting of Cr: 3.0% or less (exclusive of 0%), Mo: 1.0% or less (exclusive of 0%) and Ni: 3.0% or less (exclusive of 0%);

(b) at least one member selected from the group consisting of Cu: 2.0% or less (exclusive of 0%) and Co: 5% or less (exclusive of 0%);

(c) at least one member selected from the group consisting of V: 0.5% or less (exclusive of 0%), Ti: 0.1% or less (exclusive of 0%) and Nb: 0.1% or less (exclusive of 0%);

(d) at least one member selected from the group consisting of Ca: 0.08% or less (exclusive of 0%) and Zr: 0.08% or less (exclusive of 0%);

(e) Sb: 0.02% or less (exclusive of 0%); and

(f) at least one member selected from the group consisting of REM: 0.05% or less (exclusive of 0%), Mg: 0.02% or less (exclusive of 0%), Li: 0.02% or less (exclusive of 0%), Pb: 0.5% or less (exclusive of 0%), and Bi: 0.5% or less (exclusive of 0%).

Advantage of the Invention

According to the present invention, in a microstructure mainly containing ferrite+pearlite, grains thereof are equi-axed and refined, whereby a hot-rolled steel sheet making it possible to obtain a predetermined surface hardness after a carburizing heat treatment while ensuring cold formability can be provided.

Mode for Carrying Out the Invention

The hot-rolled steel sheet according to the present invention (hereinafter, sometimes referred to as “steel sheet of the present invention” or simply as “steel sheet”) is described in more detail below. The steel sheet of the present invention is common with the hot-forged material (high-strength high-toughness steel for case-hardening) described in Patent Document 1 in terms of the component composition but differs in that the microstructure is a microstructure mainly containing ferrite+pearlite and grains thereof are equi-axed and refined.

[Sheet Thickness of Steel Sheet of the Present Invention: from 2 to 10 mm]

The steel sheet of the present invention targets one having a sheet thickness of 2 to 10 mm. If the sheet thickness is less than 2 mm, the rigidity as a structure cannot be ensured. On the other hand, if the sheet thickness exceeds 10 mm, the microstructure configuration specified in the present invention can be hardly achieved, and the desired effects cannot be obtained. The lower limit of the sheet thickness is preferably 3 mm or more and more preferably 4 mm or more. The upper limit thereof is preferably 9 mm or less and more preferably 7 mm or less.

Next, the component composition constituting the steel sheet of the present invention is described. In the following, all of the units of chemical components are mass %.

[Component Composition of Steel Sheet of the Present Invention]

<C: from 0.05 to 0.30%>

C is an element indispensable for ensuring the strength of the core portion as a carburizing- (or carbonitriding-)quenched part finally obtained, and if it is less than 0.05%, sufficient strength cannot be obtained. However, if it is excessively contained, not only the toughness is deteriorated but also the machinability or cold forgeability is reduced to impair the formability, and therefore, the upper limit thereof is 0.30%. The preferred content of C is in the range of from 0.08 to 0.25%.

<Mn: from 0.3 to 3.0%>

Mn is an element effective for the deoxidation of molten steel and in order to effectively bring out such an effect, it must be contained in an amount of 0.3% or more. If it is excessively contained, cold formability or machinability is adversely affected and the segregation amount at grain boundary is increased to decrease the grain boundary strength, resulting in an adverse effect on the impact properties. Therefore, the content thereof must be 3.0% or less. The preferred content of Mn is in the range of from 0.5 to 2.0%.

<Al: from 0.015 to 0.1%>

Al is an element contained in the steel as a deoxidizer for the steel material and has an action of binding to N in the steel to form AN and thereby preventing grain growth. In order to effectively bring out such an effect, it must be contained in an amount of 0.015% or more. The effect is saturated at about 0.1%, and if the content exceeds it, the element binds with oxygen to form a nonmetallic inclusion and adversely affects impact properties, etc. Therefore, the upper limit thereof has been specified to be 0.1%, and is preferably 0.08% or less, more preferably 0.06% or less and especially preferably 0.04% or less.

<N: from 0.003 to 0.30%>

N has an action of binding to Al, V, Ti, Nb, etc. in the steel to form a nitride and thereby suppressing grain growth, and this effect is effectively exerted when it is contained in an amount of 0.003% or more. It is preferably 0.005% or more. However, such an effect is saturated at about 0.30%, and if contained by not less than the above amount, the nitride works as an inclusion and adversely affects the physical properties. Therefore, the upper limit has been specified to be 0.30%, and is preferably 0.10% or less, more preferably 0.05% or less and especially preferably 0.03% or less.

The steel sheet of the present invention fundamentally contains the above-described components, with the remainder being iron and unavoidable impurities. The contents of Si, P and S unavoidably getting mixed in are desirably kept as small as possible for the following reasons.

<Si: 0.5% or Less>

Si effectively acts as a strengthening element or a deoxidizing element but, on the other hand, promotes grain boundary oxidation to deteriorate the bending fatigue properties and adversely affects the cold forgeability. Accordingly, in order to remove such the problems, the content thereof must be kept at 0.5% or less and, among others, when high-level bending fatigue properties are required, the content thereof is preferably kept at 0.1% or less. From such a viewpoint, the more preferred content of Si is in the range of from 0.02 to 0.1%.

<P: 0.030% or Less>

P segregates at the grain boundary to reduce the toughness and therefore, the upper limit thereof has been specified to be 0.030%. The more preferred content of P is 0.020% or less and further preferably 0.010% or less.

<S: 0.035% or Less>

S produces MnS and contributes to enhancement of machinability. In the case of applying the present invention to gears, etc., not only vertical impact properties but also lateral impact properties are important, and the anisotropy needs to be reduced to enhance the lateral impact properties. For this purpose, the S content must be kept at 0.035% or less. The more preferred content of S is 0.025% or less and further preferably 0.020% or less.

The steel sheet of the present invention may contain the following tolerable components, in addition to the above-described basic components, in the ranges not impairing the actions of the present invention.

<At Least One Member Selected from the Group Consisting of Cr: 3.0% or Less (Exclusive of 0%), Mo: 1.0% or Less (Exclusive of 0%) and Ni: 3.0% or Less (Exclusive of 0%)>

These elements are useful elements in terms of having an action of improving the quenchability or refining the quenched microstructure. In particular, Cr has an excellent effect of enhancing the quenchability; Mo effectively acts to decrease an incompletely quenched microstructure, enhance the quenchability and furthermore, increase the grain boundary strength; and Ni refines the microstructure after quenching and thereby contributes to enhancement of impact resistance. These effects are effectively exerted by preferably containing at least one member of Cr: 0.2% or more, Mo: 0.08% or more and Ni: 0.2% or more. However, if the Cr amount exceeds 3.0%, Cr produces a carbide and causes grain boundary segregation to reduce the grain boundary strength and in turn, adversely affect the toughness; the above-described effects of Mo are saturated at about 1.0%; and the above-described effects of Ni are also saturated at 3.0%. Therefore, addition by not less than those amounts is utterly useless from an economical viewpoint.

<Cu: 2.0% or Less (Exclusive of 0%) and/or Co: 5% or Less (Exclusive of 0%)>

Cu is an element effectively acting to enhance the corrosion resistance, and the effect is effectively exerted by being contained in an amount of preferably 0.3% or more. However, the effect is saturated at a content of 2.0% and therefore, containing by not less than the above amount is useless. When Cu is contained alone, the hot formability of the steel material tends to be deteriorated and in order to avoid such an ill effect, Ni having an effect of enhancing the hot formability is desirably used in combination in the above-described content range.

In addition, both Cu and Co are elements having an action of causing strain-aging and hardening of a steel material and effective for enhancing the strength after processing. In order to effectively bring out such the actions, these elements are preferably contained each in an amount of 0.1% or more and furthermore 0.3% or more. However, if the Co content is excessively large, the effect of causing strain-aging and hardening of a steel material and the effect of enhancing the strength after processing may be saturated, or cracking may be encouraged. Therefore, it is recommended that the Co content is 5% or less, furthermore 4% or less and particularly 3% or less.

<At Least One Member Selected from the Group Consisting of V: 0.5% or Less (Exclusive of 0%), Ti: 0.1% or Less (Exclusive of 0%) and Nb: 0.1% or Less (Exclusive of 0%)>

These elements contribute to enhancing the toughness (impact resistance) by binding to C or N to produce a carbide or a nitride and by thus refining the grains. However, since the effect is saturated around each upper limit value and the machinability or cold formability may be rather adversely affected, they must be kept equal to or less than the respective upper limit values. The preferable lower limit values for effectively bringing out the addition effect of these elements are V: 0.03%, Ti: 0.005% and Nb: 0.005%.

<Ca: 0.08% or Less (Exclusive of 0%) and/or Zr: 0.08% or Less (Exclusive of 0%)>

Ca envelops a hard inclusion in a flexible inclusion, and Zr spheroidizes MnS, both thereby contributing to enhancing the machinability. In addition, both elements have an effect of increasing the lateral impact properties by virtue of the reduction in the anisotropy by the spheroidization of MnS. However, these effects are each saturated at 0.08% and therefore, it is recommended that the content of each is 0.08% or less, furthermore 0.05% or less and particularly 0.01% or less. The preferable lower limit values for effectively bringing out the above-described effects of these elements are Ca: 0.0005% (furthermore 0.001%) and Zr: 0.002%.

<Sb: 0.02% or Less (Exclusive of 0%)>

Sb is an effective element for suppressing the grain boundary oxidation and thereby increasing the bending fatigue strength. However, since the effect is saturated at 0.02%, addition by not less than the amount is useless from an economical viewpoint. The preferable lower limit value for effectively bringing out the addition effect of Sb is 0.001%.

<At Least One Member Selected from the Group Consisting of REM: 0.05% or less (exclusive of 0%), Mg: 0.02% or less (exclusive of 0%), Li: 0.02% or less (exclusive of 0%), Pb: 0.5% or less (exclusive of 0%), and Bi: 0.5% or less (exclusive of 0%)>

REM is, similarly to Zr and Ca, an element spheroidizing a sulfide compound-based inclusion such as MnS to thereby enhance the deformation performance of steel and contributing to enhancement of the machinability. In order to effectively bring out such actions, REM is preferably contained in an amount of 0.0005% or more and furthermore 0.001% or more. However, even if contained too much, the effect thereof is saturated and an effect consistent with the content cannot be expected. Therefore, recommended are 0.05% or less, furthermore 0.03% or less and particularly 0.01% or less.

The “REM” in the present invention means to include lanthanoid elements (15 elements from La to Ln) as well as Sc (scandium) and Y (yttrium). Among these elements, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y, and it is more preferable to contain La and/or Ce.

Mg is, similarly to Zr and Ca, an element spheroidizing a sulfide compound-based inclusion such as MnS to thereby enhance the deformation performance of steel and contributing to enhancement of the machinability. In order to effectively bring out such actions, Mg is preferably contained in an amount of 0.0002% or more and furthermore 0.0005% or more. However, even if contained too much, the effect thereof is saturated and an effect consistent with the content cannot be expected. Therefore, recommended are 0.02% or less, furthermore 0.015% or less and particularly 0.01% or less.

Li is, similarly to Zr and Ca, an element spheroidizing a sulfide compound-based inclusion such as MnS to allow for enhancement of the deformation performance of steel and contributing to improvement of the machinability by lowering the melting point of an Al-based oxide and thereby making it harmless. In order to effectively bring out such actions, Li is preferably contained in an amount of 0.0002% or more and furthermore 0.0005% or more. However, even if contained too much, the effect thereof is saturated and an effect consistent with the content cannot be expected. Therefore, recommended are 0.02% or less, furthermore 0.015% or less and particularly 0.01% or less.

Pb is an element effective for enhancing the machinability. In order to effectively bring out such an action, Pb is preferably contained in an amount of 0.005% or more and furthermore 0.01% or more. However, if contained too much, there arises a problem with production such as formation of a roll mark. Therefore, recommended are 0.5% or less, furthermore 0.4% or less and particularly 0.3% or less.

Bi is, similarly to Pb, an element effective for enhancing the machinability. In order to effectively bring out such an action, Bi is preferably contained in an amount of 0.005% or more and furthermore 0.01% or more. However, even if contained too much, the effect of enhancing the machinability is saturated. Therefore, recommended are 0.5% or less, furthermore 0.4% or less and particularly 0.3% or less.

The microstructure characterizing the steel sheet of the present invention is described below.

[Microstructure of Steel Sheet of the Present Invention]

As described above, the steel sheet of the present invention has ferrite+pearlite as a principal microstructure and in particular, is characterized in that with respect to grains of all phases including ferrite and pearlite, each of the equiaxing degree and the size is controlled to a specific range.

<Microstructure Containing Ferrite: from 10 to 50%, Pearlite: from 15 to 50% and Remainder: Bainite>

The percentage of a phase is an important factor in determining the strength level of the steel sheet. In the present invention, from the standpoint of ensuring cold formability and the matrix strength in the central part of sheet thickness after a heat treatment, it is necessary to be set to approximately from 350 to 700 MPa as a tensile strength. If the tensile strength is less than 350 MPa, although the surface hardness may be ensured even after a carburizing heat treatment, the hardness in a deep portion from the surface is insufficient, and the strength and hardness in the central part of sheet thickness are insufficient. On the other hand, if the tensile strength exceeds 700 MPa, the cold formability before a heat treatment cannot be ensured. In correspondence with the strength level, if the percentage of ferrite is too small and/or the percentage of pearlite is too large, the tensile strength is excessively increased to make the forming impossible. On the other hand, if the percentage of ferrite is too large and/or the percentage of pearlite is too small, the base strength lacks, leading to insufficient strength in the central part of sheet thickness and a decrease in the fatigue strength. Therefore, the microstructure is specified to contain, in area ratio, ferrite: from 10 to 50% and pearlite: from 15 to 50%. The remainder is bainite.

<The Number of Grains Having an Aspect Ratio (Major Axis/Minor Axis) of 3 or Less is 60% or More of the Number of All Grains>

The shape of the grain is required to be equi-axed grain for achieving both enhancement of the stretch flange formability (hole expandability) and assurance of the hardness, after a heat treatment, in the central part of sheet thickness, i.e., in a deep portion from the surface. For this purpose, the number of grains having an aspect ratio (major axis/minor axis) of 3 or less, which are an equi-axed grain, is set to be 60% or more, preferably 70% or more and more preferably 80% or more, of the number of all grains. The “all grains” herein means grains of all phases including the above-described ferrite and pearlite.

<The Average Grain Size of All Grains is in the Range of from 3 to 50 μm>

If the grain is too large, the surface quality is degraded to cause surface cracking and to deteriorate the hole expandability. Therefore, the average grain size of all grains is set to be 50 μm or less, preferably 40 μm or less and more preferably 30 μm or less. On the other hand, in regard to the lower limit value, as the grain is finer, the properties thereof are improved, but the rolling capacity or cooling capacity must be raised, which reduces the productivity. Therefore, the average grain size of all grains is set to be 3 μm or more, preferably 5 μm or more and more preferably 7 μm or more.

[Method for Measuring Area Ratio of Each Phase]

As for the area ratio of each phase above, each test steel sheet is ground to a depth of t/4 (t: sheet thickness), then subjected to Nital etching, and photographed for five visual fields by a scanning electron microscope (SEM, magnification: 1,000 times), thereby determining each percentage of ferrite and pearlite by a point counting method. The remainder was regarded as bainite.

[Method for Measuring Aspect Ratio of Grain]

Grains of all phases including the above-described ferrite and pearlite are measured for the maximum Feret diameter and the minimum Feret diameter, and the ratio thereof (major axis/minor axis) was defined as the aspect ratio.

[Method for Measuring Average Grain Size]

Individual centroid diameters as for all grains are determined by performing image analysis on the above images photographed by the scanning electron microscope, and a value obtained by arithmetically averaging these centroid diameters by the number of all grains is defined as the average grain size of all grains.

The preferable production method for obtaining the steel sheet of the present invention is described below.

[Preferable Production Method of Steel Sheet of the Present Invention]

The steel sheet of the present invention can be produced, for example, as a hot-rolled coil obtained by melting a raw material steel having the above-described component composition, casting it to form a slab, and subjecting the slab as it is or the slab after surface chamfering to respective steps of heating, hot rough rolling and finish rolling. Pickling and skin pass rolling may be thereafter further applied according to the required conditions such as surface state and sheet thickness accuracy.

[Preparation of Molten Steel]

First, desired oxides can be produced by adding predetermined alloy elements in a predetermined order to a molten steel in which the dissolved oxygen amount and the total oxygen amount are adjusted. Above all, in the present invention, it is very important to adjust the dissolved oxygen amount and thereafter, adjust the total oxygen amount so as to inhibit production of a coarse oxide.

The “dissolved oxygen” means oxygen that is present in the molten steel without forming an oxide and kept in a free state. The “total oxygen” means the total of all oxygens contained in the molten steel, i.e., free oxygen and oxygen forming an oxide.

The dissolved oxygen amount in the molten steel is first adjusted to a range of 0.0010 to 0.0060%. If the dissolved oxygen amount in the molten steel is less than 0.0010%, a predetermined amount of Al—O-based oxide cannot be ensured due to shortage of the dissolved oxygen amount in the molten steel, and a desired size distribution cannot be obtained. In addition, if the dissolved oxygen amount is insufficient, in the case of adding REM, the REM forms a sulfide, and an inclusion is thereby coarsened, giving rise to deterioration of the properties. Therefore, the dissolved oxygen amount is set to be 0.0010% or more. The dissolved oxygen amount is preferably 0.0013% or more and more preferably 0.0020% or more.

On the other hand, if the dissolved oxygen amount exceeds 0.0060%, not only the reaction of oxygen and the elements above in the molten steel becomes vigorous due to an excessively large oxygen amount in the molten steel, which is disadvantageous in view of melting operation, but also a coarse oxide is produced to rather deteriorate the properties. Therefore, the dissolved oxygen amount should be kept at 0.0060% or less. The dissolved oxygen amount is preferably 0.0055% or less and more preferably 0.0053% or less.

The dissolved oxygen amount in a molten steel having been subjected to primary refining in a converter or an electric furnace usually exceeds 0.010%. Therefore, in the production method of the present invention, the dissolved oxygen amount in the molten steel needs to be adjusted to the range above in some way.

The method for adjusting the dissolved oxygen amount in the molten steel includes, for example, a method of performing vacuum C deoxidation by using an RH-type degassing refining apparatus and a method of adding a deoxidizing element such as Si, Mn and Al, and the dissolved oxygen amount may also be adjusted by appropriately combining these methods. In addition, the dissolved oxygen amount may be adjusted by using a ladle heating-type refining apparatus, a simple molten metal treatment system, etc., in place of the RH-type degassing refining apparatus. In this case, since the dissolved oxygen amount cannot be adjusted by vacuum C deoxidation, a method of adding a deoxidizing element such as Si may be employed for the adjustment of the dissolved oxygen amount. In the case of employing the method of adding a deoxidizing element such as Si, the deoxidizing element may be added when the steel is tapped from the converter to the ladle.

After adjusting the dissolved oxygen amount in the molten steel to the range of 0.0010 to 0.0060%, the molten steel is stirred to float and separate an oxide in the molten steel, whereby the total oxygen amount in the molten steel is adjusted to the range of 0.0010 to 0.0070%. Thus, in the present invention, after removing unnecessary oxides by stirring a molten steel in which the molten oxygen amount is appropriately controlled, the production of a coarse oxide, i.e., a coarse inclusion, can be prevented.

If the total oxygen amount is less than 0.0010%, the desired amount of oxide lacks and therefore, the amount of oxide contributing to a fine size distribution of inclusions cannot be ensured. Therefore, the total oxygen amount is set to be 0.0010% or more. The total oxygen amount is preferably 0.0015% or more and more preferably 0.0018% or more.

On the other hand, if the total oxygen amount exceeds 0.0070%, the amount of oxide in the molten steel is excessively large, as a result, a coarse oxide, i.e., a coarse inclusion, is produced to deteriorate the properties. Therefore, the total oxygen amount should be kept at 0.0070% or less. The total oxygen amount is preferably 0.0060% or less and more preferably 0.0050% or less.

The total oxygen amount in the molten steel generally varies in a correlated manner in response to the stirring time of the molten steel and therefore, can be controlled, for example, by adjusting the stirring time. Specifically, the total oxygen amount in the molten steel is appropriately controlled while stirring the molten steel and measuring from time to time the total oxygen amount in the molten steel after removing an oxide floated.

In the case of adding REM and Ca to the steel material, after adjusting the total oxygen amount in the molten steel to the above-described range, REM is added and casting is then performed. A desired oxide can be obtained by adding the elements above to a molten steel in which the total oxygen amount has been adjusted.

The forms of REM and Ca to be added to the molten steel are not particularly limited and, for example, pure La, pure Ce, pure Y, etc. as REM, or pure Ca, and furthermore, Fe—Si—La alloy, Fe—Si—Ce alloy, Fe—Si—Ca alloy, Fe—Si—La—Ce alloy, Fe—Ca alloy, or Ni—Ca alloy may be added. A misch metal may also be added to the molten metal. The misch metal is a mixture of cerium group rare-earth elements and, specifically, contains approximately from 40 to 50% of Ce and approximately from 20 to 40% of La. However, the misch metal often contains Ca as an impurity and in the case where the misch metal contains Ca, the preferable range specified in the present invention must be satisfied.

In the case where REM is added, in the present invention, the molten steel after the addition of REM is preferably stirred for in the range of not more than 40 minutes so as to promote the removal of a coarse oxide. If the stirring time exceeds 40 minutes, an oxide is coarsened due to occurrence of aggregation/coalescence of fine oxides in the molten steel, whereby the properties are deteriorated. Therefore, the stirring time is preferably 40 minutes or less. The stirring time is more preferably 35 minutes or less and still more preferably 30 minutes or less. The lower limit value of the stirring time of the molten steel is not particularly limited, but if the stirring time is too short, the concentrations of additive elements are non-uniform, and the desired effect as the entire steel material cannot be obtained. Accordingly, a desired stirring time in accordance with the container size is required.

In this way, a molten steel having an adjusted component composition can be obtained. By using the obtained molten steel, casting is performed to obtain a billet.

Next, heating, hot rolling including finish rolling, rapid cooling after hot rolling, slow cooling after stop of rapid cooling, rapid cooling and coiling after slow cooling are performed for the production.

[Heating]

The heating before hot rolling is performed at 1,150 to 1,300° C. This heating provides for an austenite single phase, whereby a solid solution element (including an additive element such as V and Nb) is dissolved in solid in the austenite. If the heating temperature is less than 1,150° C., the element cannot be dissolved in solid in the austenite, and a coarse carbide is formed, as a result, an effect of improving the fatigue properties cannot be obtained. On the other hand, a temperature exceeding 1,300° C. is difficult in view of operation. In the case of containing Ti as an additive element, from the standpoint of forming a solid solution of Ti, which has a highest solution treatment temperature among carbides, a temperature equal to or more than the solution treatment temperature of TiC and 1,300° C. or less is necessary. The more preferred lower limit of the heating temperature is 1,200° C.

[Hot Rough Rolling]

In the rough rolling, the microstructure control of recrystallized austenite is performed so as to ensure the presence percentage of an equiaxial grain with a predetermined shape specified in the present invention. Taking into account to ensure the temperature in the subsequent finish rolling, the rough rolling temperature is set to be from 900 to 1,200° C., and the austenite grain is refined and repeatedly recrystallized in the rough rolling, whereby the presence percentage of the equiaxial grain with a predetermined shape can be controlled. The rough rolling temperature is more preferably from 900 to 1,100° C.

[Hot Finish Rolling]

Hot rolling is performed so that the finish rolling temperature can be 800° C. or more. If the finish rolling temperature is set too low, ferrite transformation occurs at a high temperature, and a precipitated carbide in ferrite is coarsened. Therefore, the finish rolling temperature needs to be not less than a given level. The finish rolling temperature is more preferably set to be 850° C. or more so that the austenite grain can grow and the grain size of bainite can be increased.

[Difference Between Entry-Side Temperature and Exit-Side Temperature of Hot Finish Rolling]

The difference between the entry-side temperature and the exit-side temperature of hot finish rolling is set to be 150° C. or less. In the case where this temperature difference exceeds 150° C., which is the case where the temperature before finish rolling is high, not only the grain (austenite grain) grows but also the recrystallized grain produced during finish rolling readily grows large. In addition, when the temperature difference between the entry side and the exit side is large, the recrystallized microstructure produced during finish rolling is likely to become non-uniform, and a grain having a large aspect ratio tends to remain. For these reasons, the number of grains having an aspect ratio of 3 or less becomes less than 60% of the number of all grains. The temperature difference is more preferably 100° C. or less.

[Rapid Cooling After Hot Rolling]

Within 5 seconds after the completion of the finish rolling, rapid cooling is performed at a cooling rate (rapid cooling rate) of 20° C./s or more, and the rapid cooling is stopped at a temperature (rapid cooling stop temperature) of 580° C. or more and less than 680° C. This is done for lowering the ferrite transformation start temperature and thereby refining the precipitated carbide formed in ferrite. If the cooling rate (rapid cooling rate) is less than 20° C./s, pearlite transformation is promoted, or if the rapid cooling stop temperature is less than 580° C., pearlite transformation or bainite transformation is promoted and, as a result, cold formability is reduced. On the other hand, if the rapid cooling stop temperature is 680° C. or more, the precipitated carbide in ferrite is coarsened, and the anti-fatigue properties cannot be ensured. The rapid cooling stop temperature is preferably from 600 to 650° C. and more preferably 610 to 640° C.

[Slow Cooling After Stop of Rapid Cooling]

After the stop of the rapid cooling, slow cooling is performed at a cooling rate (slow cooling rate) of 5° C./s or more and less than 20° C./s. The slow cooling rate is set to be 5° C./s or more so as to suppress formation of proeutectoid ferrite during hot rolling, appropriately refine the precipitated carbide in ferrite, and control the grain microstructure in the hot-rolled sheet, thereby controlling the texture configuration in the final steel sheet.

If the slow cooling rate is less than 5° C./s, not only the amount of proeutectoid ferrite formed is increased, allowing production of a coarse grain, but also a coarse grain is formed in the final steel sheet to cause a non-uniform state of carbide and deteriorate the cold formability. If the cooling rate is 20° C./s or more, martensite is produced, and the cold formability is thereby reduced.

[Rapid Cooling and Coiling After Slow Cooling]

After the slow cooling, coiling is performed at more than 550° C. and 650° C. or less. If the coiling temperature exceeds 650° C., many surface oxide scales are formed to deteriorate the surface quality, and on the other hand, if it is less than 550° C., many martensites are formed to reduce the cold formability.

The present invention is described in greater detail below by referring to Examples, but the following Examples are not limiting in nature on the present invention, and the present invention can be implemented by making appropriate changes within the scope conformable to the gist described hereinbefore and hereinafter, and all of them are included in the technical scope of the present invention.

EXAMPLES

A steel having the component composition shown in Table 1 below was melted by a vacuum melting method and cast into an ingot having a thickness of 120 mm, followed by performing hot rolling under the conditions shown in Table 2 below to produce a hot-rolled steel sheet. In all tests, the cooling after the stop of rapid cooling was slow cooling under the conditions of a cooling rate of 10° C./s or less for 5 to 20 seconds.

A test steel containing the chemical components shown in Table 1 was melted by using a vacuum melting furnace (capacity: 150 kg) and cast into 150 kg of an ingot, followed by cooling. When the test steel was melted in the vacuum melting furnace, component adjustment was applied to the elements except for Al, REM and Ca, and the dissolved oxygen amount in the molten steel was adjusted by deoxidation by using at least one element selected from C, Si and Mn. The molten steel in which the dissolved oxygen amount is adjusted was stirred for approximately from 1 to 10 minutes to float and separate oxides in the molten steel, and the total oxygen amount in the molten steel was thereby adjusted.

In the case of adding REM and Ca, they were added to a molten steel in which the total oxygen amount had been adjusted, thereby obtaining a molten steel adjusted for component thereof. Here, REM was added in the form of a misch metal containing about 25% of La and about 50% Ce, and Ca was added in the form of an Ni—Ca alloy, a Ca—Si alloy, or an Fe—Ca green compact.

The obtained ingot was hot-rolled under the conditions shown in Table 2 to produce a hot-rolled sheet having a predetermined thickness. In Table 2, the rapid cooling rate after hot rolling and the cooling rate after stop of rapid cooling are not shown, but in each production example, the conditions employed are 40° C./s for the rapid cooling after hot rolling and 10° C./s for the cooling after stop of rapid cooling.

With respect to each of the thus-obtained as-hot rolled sheets, the area ratio of each phase in the steel sheet, the aspect ratio of the grain, the number thereof, etc. were examined by the measuring methods described in the item of “MODE FOR CARRYING OUT THE INVENTION” above.

In addition, each of the as-hot rolled sheets above was measured for the tensile strength and the hole expansion ratio for evaluating the cold formability, and those where the tensile strength was in the range of from 350 to 700 MPa and the hole expansion ratio was 20% or more were judged as passed.

Furthermore, each of the as-hot rolled sheets above was subjected to a carburizing-quenching test under the following conditions for evaluating the surface hardness after a carburizing heat treatment.

[Carburizing-Quenching Conditions]

A carburizing treatment was applied by holding at 900° C. for 2.5 hours and further at 850° C. for 0.5 hours in a gas atmosphere with a carbon potential (CP value)=0.8% and thereafter, performed were oil-quenching at 100° C., holding at 160° C. for 2 hours for subjecting to a tempering treatment, and air-cooling.

<Surface Hardness After Carburizing Heat Treatment>

The Vickers hardness (Hv) was measured by using a Vickers hardness tester under the conditions of load: 1,000 g, measurement position: a position of 0.8 mm in depth from the steel sheet surface, and number of measurements: 5 times, and those where the hardness was 350 Hv or more were judged as passed. Here, the measurement position was set to a position of 0.8 mm in depth from the surface because it was specified as a necessary condition to exhibit desired hardness (strength) even at a deep position from the surface after a carburizing heat treatment.

These measurement results are shown in Table 3 below.

TABLE 1 Steel Spe- Components (mass %) [remainder: Fe and unavoidable impurities] cies C Si Mn P S Al N Cr Mo Ni Cu Co V Ti Nb Ca Zr Sb Others a 0.23 0.05 2.50 0.008 0.012 0.032 0.010 — — — — — — — — — — — — b 0.08 0.10 2.10 0.010 0.018 0.026 0.005 — — — — — — — — — — — — c 0.15 0.06 1.88 0.008 0.012 0.027 0.090 1.03 — — — — — — — — — — — d 0.13 0.02 2.20 0.012 0.020 0.016 0.008 0.99 — — — — — — — — — — — e 0.17 0.12 2.01 0.010 0.021 0.022 0.009 — 0.44 — — — — — — — — — — f 0.21 0.41 1.32 0.010 0.008 0.033 0.010 — — 0.99 — — — — — — — — — g 0.17 0.05 1.40 0.008 0.012 0.032 0.010 1.07 0.01 0.02 0.01 — — — — — — — — h 0.19 0.07 1.52 0.010 0.027 0.022 0.080 1.01 — 0.28 — 0.32 — — — — — — — i 0.18 0.06 1.39 0.009 0.010 0.03  0.090 — — — — — 0.33 — — — — — — j 0.19 0.06 1.41 0.010 0.011 0.031 0.070 — — — — — — 0.05 — — — — — k 0.17 0.07 1.40 0.008 0.012 0.03  0.090 — — — — — — — 0.03 — — — — l 0.18 0.06 1.38 0.007 0.010 0.029 0.010 1.03 0.02 0.02 0.01 — — — — 0.002 — — — m 0.18 0.05 1.39 0.007 0.010 0.029 0.010 0.99 0.02 0.03 — 0.01 — — — — 0.002 — — n 0.17 0.05 1.41 0.007 0.009 0.032 0.080 1.07 0.01 0.02 0.01 — — — — — — 0.008 — o 0.17 0.05 1.41 0.007 0.009 0.032 0.080 1.07 0.01 0.02 0.01 — — — — — — — REM: 0.003, Li: 0.001 p 0.19 0.06 1.36 0.007 0.009 0.029 0.070 1.01 0.01 0.02 0.01 — — — — — — — Mg: 0.001, Pb: 0.001, Bi: 0.05 q 0.03 0.05 2.20 0.008 0.001 0.03  0.009 — — — — — — — — — — — — r 0.31 0.05 2.20 0.007 0.001 0.03  0.009 — — — — — — — — — — — — s 0.17 0.07 0.20 0.007 0.001 0.03  0.009 — — — — — — — — — — — — t 0.17 0.07 3.15 0.007 0.001 0.03  0.009 — — — — — — — — — — — — u 0.17 0.07 2.10 0.007 0.001 0.009 0.009 — — — — — — — — — — — — v 0.17 0.07 2.10 0.007 0.001 0.11  0.009 — — — — — — — — — — — — w 0.17 0.07 2.10 0.007 0.001 0.025 0.310 — — — — — — — — — — — — (—: not added, underlined: outside the scope of the present invention)

TABLE 2 Hot Rolling Conditions Rough Finish Rolling Finish Rolling Temperature Rapid Thickness of Steel Heating Rolling Entry-Side Exit-Side Difference Between Cooling Stop Coiling Hot-Rolled Production Spe- Temperature Temperature Temperature Temperature Entry Side − Exit Side Temperature Temperature Sheet No. cies (° C.) (° C.) (° C.) (° C.) (° C.) (° C.) (° C.) (mm)  1 a 1250 1121 987 880 107  601 569 3  2 a 1250 1056 991 912 79 612 598 8  3* a  1000* 920 864  775* 89  532*  457* 5  4* a 1250 1086 975 872 103  655 586 12*  5* a 1250 1189 997 831 166* 629 563 5  6 b 1250 1101 942 880 62 602 556 5  7 c 1250 1193 924 875 49 673 576 5  8 d 1250 964 935 864 71 642 579 5  9 e 1250 1151 952 890 62 657 598 5 10 f 1250 1113 923 855 68 635 568 5 11 g 1250 1199 970 843 127  638 581 5 12 h 1250 1183 925 856 69 643 576 5 13 i 1250 1025 943 880 63 644 552 5 14 j 1250 1000 891 867 24 603 558 5 15 k 1250 967 923 879 44 643 609 5 16 l 1250 1025 911 890 21 655 562 5 17 m 1250 1163 903 868 35 624 622 5 18 n 1250 986 966 901 65 657 596 5 19 o 1250 1038 913 867 46 615 601 5 20 p 1250 1102 911 850 61 615 567 5 21 q 1250 1021 931 844 87 622 571 5 22 r 1250 1169 947 897 50 603 601 5 23 s 1250 1120 962 876 86 620 558 5 24 t 1250 1146 876 843 33 607 579 5 25 u 1250 1021 953 901 52 620 582 5 26 v 1250 1048 924 893 31 604 559 5 27 w 1250 1039 923 851 72 648 612 5 (underlined = outside the scope of the present invention, *= outside the recommended range)

TABLE 3 Ratio of Number Surface Hardness Area Ratio of of Grains with Cold Formability After Carburizing Steel Each Phase Aspect Ratio of Average Tensile Hole Expansion Hardness at Steel Spe- Production Ferrite Pearlite Bainite 3 or Less Grain Size Strength Ratio Depth of 0.8 mm No. cies No. (%) (%) (%) (%) (μm) (MPa) (%) (Hv) Remarks 1 a  1 42 45 13 73 28 583 34 390 Steel of Invention 2 a  2 43 42 15 66 41 541 33 361 Steel of Invention 3 a  3* 20 70 10 42 19 723 11 394 Comp. steel 4 a  4* 55 39 6 68 58 523 17 386 Comp. steel 5 a  5* 47 45 8 53 18 584 18 398 Comp. steel 6 b  6 38 32 30 74 25 594 31 403 Steel of Invention 7 c  7 39 36 25 72 24 587 32 406 Steel of Invention 8 d  8 41 29 30 72 22 576 28 411 Steel of Invention 9 e  9 40 30 30 70 22 583 29 405 Steel of Invention 10 f 10 37 29 34 71 21 591 28 403 Steel of Invention 11 g 11 42 45 13 79 15 601 28 422 Steel of Invention 12 h 12 44 37 19 89 16 595 29 413 Steel of Invention 13 i 13 42 38 20 84 18 600 31 411 Steel of Invention 14 j 14 39 45 16 72 22 645 28 399 Steel of Invention 15 k 15 38 44 18 89 23 645 33 403 Steel of Invention 16 l 16 41 46 13 89 21 588 31 415 Steel of Invention 17 m 17 43 37 20 81 24 566 36 401 Steel of Invention 18 n 18 45 33 22 89 23 638 35 403 Steel of Invention 19 o 19 36 35 29 82 21 566 33 404 Steel of Invention 20 p 20 37 39 24 79 26 605 31 406 Steel of Invention 21 q 21 78 11 11 86 46 320 51 246 Comp. steel 22 r 22 11 74 15 43 26 853 — 460 Comp. steel 23 s 23 61 13 26 44 24 420 35 294 Comp. steel 24 t 24  9 66 25 15 24 687 17 411 Comp. steel 25 u 25 42 44 14 13 23 590 15 407 Comp. steel 26 v 26 41 42 17 12 21 603 13 406 Comp. steel 27 w 27 46 47 7 14 26 602 17 402 Comp. steel (underlined = outside the scope of the present invention, *= outside the recommended range, —: not measured due to generation of cracking)

As shown in Table 3, all of Steel Nos. 1, 2 and 6 to 20 are Steels of the Invention produced by using a steel species satisfying the requirements specified for the component composition of the present invention under the recommended hot rolling conditions, as a result, allowed to satisfy the requirements specified for the microstructure of the present invention, and in the steels, all of the tensile strength, the hole expansion ratio and the surface hardness after a carburizing heat treatment meet the acceptance criteria. It could be confirmed that a hot-rolled steel sheet exhibiting a predetermined hardness (strength) after a carburizing heat treatment while ensuring good cold formability can be obtained.

On the other hand, Steel Nos. 3 to 5 and 21 to 27 are Comparative Steels failing in satisfying at least one of the requirements specified for the component composition and the microstructure in the present invention, and in the steels, at least one of the tensile strength, the hole expansion ratio and the surface hardness after a carburizing heat treatment does not meet the acceptance criteria.

For example, Steel No. 3 satisfies the requirements for the component composition, but since the heating temperature before hot rolling is outside the recommended range and too low, pearlite is excessively formed and the grain is flattened, resulting in too high tensile strength and poor hole expandability.

Steel No. 4 satisfies the requirements for the component composition, but since the sheet thickness after hot rolling is outside the specified range and too large, ferrite is excessively formed and the grain grows, resulting in poor hole expandability.

Steel No. 5 satisfies the requirements for the component composition, but since the difference between the entry-side temperature and the exit-side temperature in finish rolling is outside the recommended range and too large, the grain is flattened, resulting in poor hole expandability.

In Steel No. 21 (steel species q), the hot rolling conditions are in the recommended range but the C content is too low, and therefore ferrite is excessively formed, and the tensile strength is excessively decreased resulting in poor surface hardness after a carburizing heat treatment.

In Steel No. 22 (steel species r), the hot rolling conditions are in the recommended range but the C content is too high, and therefore pearlite is excessively formed and the grain is flattened, resulting in too high tensile strength and generation of cracking at the time of a hole expansion test (poor hole expandability).

In Steel No. 23 (steel species s), the hot rolling conditions are in the recommended range but the Mn content is too low, and therefore ferrite is excessively formed and the grain is flattened, resulting in poor surface hardness after a carburizing heat treatment.

In Steel No. 24 (steel species t), the hot rolling conditions are in the recommended range but the Mn content is too high, and therefore ferrite is not sufficiently formed while pearlite is excessively formed, and in addition, the ratio of the number of grains having an aspect ratio of 3 or less is low, resulting in poor hole expandability.

In Steel No. 25 (steel species u), the hot rolling conditions are in the recommended range but the Al content is too low and the ratio of the number of grains having an aspect ratio of 3 or less is low, resulting in poor hole expandability.

On the other hand, in Steel No. 26 (steel species v), the hot rolling conditions are in the recommended range but the Al content is too high and the ratio of the number of grains having an aspect ratio of 3 or less is low, resulting in poor hole expandability as well.

In Steel No. 27 (steel species w), the hot rolling conditions are in the recommended range but the N content is too low and the ratio of the number of grains having an aspect ratio of 3 or less is low, resulting in poor hole expandability.

From the above, the applicability of the present invention could be confirmed.

While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the present invention.

This application is based on Japanese Patent Application (Patent Application No. 2013-219467) filed on Oct. 22, 2013, the contents of which are incorporated herein by way of reference.

INDUSTRIAL APPLICABILITY

The hot-rolled steel sheet of the present invention exhibits good cold formability during processing, and, after a carburizing heat treatment, is excellent in the hardness on the surface as well as at a predetermined depth and excellent in the wear resistance, anti-fatigue properties, etc., and therefore, is useful as materials for the production of clutches, dampers, gears, etc. of automobiles. 

1. A hot-rolled steel sheet excellent in cold formability and surface hardness after a carburizing heat treatment, having: a sheet thickness of from 2 to 10 mm; a component composition comprising, in mass % (hereinafter, the same applies to chemical components), C: from 0.05 to 0.30%, Mn: from 0.3 to 3.0%, Al: from 0.015 to 0.1%, and N: from 0.003 to 0.30%, with the remainder being iron and unavoidable impurities; and a microstructure comprising, in area ratio, ferrite: from 10 to 50%, pearlite: from 15 to 50% and remainder: bainite, wherein with respect to grains of all phases including the ferrite and the pearlite (hereinafter, referred to as “all grains”), a number of grains having an aspect ratio (major axis/minor axis) of 3 or less is 60% or more of a number of the all grains, and an average grain size of the all grains is from 3 to 50 μm.
 2. The hot-rolled steel sheet according to claim 1, wherein, in the unavoidable impurities, Si is 0.5% or less, P is 0.030% or less and S is 0.035% or less.
 3. The hot-rolled steel sheet according to claim 1, wherein the component composition further comprises at least any one of the following (a) to (f): (a) at least one member selected from the group consisting of Cr: 3.0% or less (exclusive of 0%), Mo: 1.0% or less (exclusive of 0%) and Ni: 3.0% or less (exclusive of 0%); (b) at least one member selected from the group consisting of Cu: 2.0% or less (exclusive of 0%) and Co: 5% or less (exclusive of 0%); (c) at least one member selected from the group consisting of V: 0.5% or less (exclusive of 0%), Ti: 0.1% or less (exclusive of 0%) and Nb: 0.1% or less (exclusive of 0%); (d) at least one member selected from the group consisting of Ca: 0.08% or less (exclusive of 0%) and Zr: 0.08% or less (exclusive of 0%); (e) Sb: 0.02% or less (exclusive of 0%); and/or (f) at least one member selected from the group consisting of REM: 0.05% or less (exclusive of 0%), Mg: 0.02% or less (exclusive of 0%), Li: 0.02% or less (exclusive of 0%), Pb: 0.5% or less (exclusive of 0%), and Bi: 0.5% or less (exclusive of 0%).
 4. The hot-rolled steel sheet according to claim 2, wherein the component composition further comprises at least any one of the following (a) to (f): (a) at least one member selected from the group consisting of Cr: 3.0% or less (exclusive of 0%), Mo: 1.0% or less (exclusive of 0%) and Ni: 3.0% or less (exclusive of 0%); (b) at least one member selected from the group consisting of Cu: 2.0% or less (exclusive of 0%) and Co: 5% or less (exclusive of 0%); (c) at least one member selected from the group consisting of V: 0.5% or less (exclusive of 0%), Ti: 0.1% or less (exclusive of 0%) and Nb: 0.1% or less (exclusive of 0%); (d) at least one member selected from the group consisting of Ca: 0.08% or less (exclusive of 0%) and Zr: 0.08% or less (exclusive of 0%); (e) Sb: 0.02% or less (exclusive of 0%); and/or (f) at least one member selected from the group consisting of REM: 0.05% or less (exclusive of 0%), Mg: 0.02% or less (exclusive of 0%), Li: 0.02% or less (exclusive of 0%), Pb: 0.5% or less (exclusive of 0%), and Bi: 0.5% or less (exclusive of 0%). 